1. Field of the Invention
The present invention relates generally to a noise correction in an image sensor, and more particularly to row and/or column noise calibration and removal in image sensor 100 chips.
2. Description of the Related Art
An image sensor typically comprises an array of photosensitive elements, which are often called pixels. The pixels are exposed to light under the control of various timing and/or control circuits. To reduce noise in the system, many image sensor designs use the method of correlated double sampling (CDS). In CDS, the photosensitive elements are first reset, and the reset signal of each pixel is read from the array. This is usually referred to as the reset value, P-phase value, or cds value. The array is then exposed to light and the light exposed value of each pixel is read. This is usually referred to as the light exposed value, D-phase value, or data value. The cds value of each pixel is then subtracted from the data value of the same pixel to produce the output value of the pixel. For image sensors that do not use CDS, only the light exposed values are read from the pixels to produce the output.
While the CDS method is effective in reducing or removing the noise due to the pixels, it does not remove noise in the other circuit elements in the sensor such as the readout circuit, the sample-and-hold, the analog-to-digital converter, etc. In particular, noise is added to the image signal that is outputted from the image sensor 100 during the read out process, and this noise introduces unwanted artifacts that degrade image quality and lead to visible image defects.
It is important to distinguish between temporal noise and fixed pattern noise. While both types of noise can be reduced or removed using methods of calibration, correction and/or filtering, the method that works for mitigating one type of noise may not work for mitigating the other type of noise. Generally fixed pattern noise can be mitigated using a calibration method prior to an imaging operation—for example, during power up of the image sensor—and the results of the calibration can be stored and used later during imaging to correct the pixel data.
However, this approach of performing calibration in advance of imaging and subsequently using stored calibration values cannot be effectively used to mitigate noise that is temporal in nature. Instead, effective mitigation of temporal noise may require performing noise mitigation processes repeatedly during operation of the image sensor, since the noise may vary with time.
Various approaches to compensating for fixed pattern and temporal noise involve providing special means for measuring the noise and then generating values that are used to compensate for the noise. For example, U.S. Pat. No. 6,433,822 discloses an image sensor including a calibration reference circuit and calibration logic, which together produce and store a reference signal for each column that is later subtracted from pixel data that has been read out to produce corrected data. U.S. Pat. No. 8,237,824 corrects fixed pattern noise by generating a per-pixel calibration model for each pixel in the sensor array using measurements under a plurality of illumination levels; the model is then used for correcting the output pixel values in image capture. U.S. Pat. No. 6,438,695 uses a set of calibration pixels (which are different from the actual pixels) to capture image data, injects reference voltages into the calibration pixels to measure the characteristics of the circuit readout elements (e.g. the bitlines) and generates calibration data based on the measurements, which is used to correct the actual image pixels. U.S. Pat. No. 8,817,120 and US patent application 2013/0321671A1 collects statistics from a frame of data (image frame) and determines correction factors (such as gain and offset values) based on the statistics for the purpose of correcting the rows and columns.